For what it is worth, I would like to put in my vote for Richard Strauss.
It seems to me that works like Salome and Elektra heralded a new aesthetic
for this century; even more so than Stravinsky who I think would be the
only other contender (except perhaps for Gershwin or Schoenberg!) The
problem with Stravinsky is that he never again displayed the genius of
the Rite of Spring, Petroushka or the Firebird; at least not to my taste.
Whereas Strauss went on to write many other great operas (Rosenkavalier,
Die Frau ohne Schatten, Ariadne auf Naxos etc), and the ineffable Vier
letzte Lieder and the magnificent Clarinet concerto.  These works were very
different from the two earlier operas but they are still works of genius.
I think no other composer has written so much of real worth in this
century.  Not to mention what he wrote in the previous one!

Gerry Kelly